1. Field of the Invention
The present invention relates to a sputtering target for fabricating a recording layer of an optical recording medium; a method of producing the sputtering target; a phase-change type optical recording medium comprising a recording material in a recording layer thereof, which recording material is capable of causing changes in the phase thereof by the application of a laser beam thereto, thereby recording, reproducing and overwriting information therein; and a method of fabricating the above-mentioned phase-change type optical recording medium by using the above-mentioned sputtering target.
2. Discussion of Background
There is conventionally known a phase-change type optical information recording medium which utilizes phase changes between a crystalline phase and an amorphous phase or between one crystalline phase and another crystalline phase as one of the optical recording media which are capable of recording, reproducing and erasing information by the application thereto of electromagnetic waves, such as a laser beam. This kind of phase-change type optical information recording medium enables the overwriting of information by the application of a single laser beam thereto, although such overwriting is difficult to conduct by the application of a single laser beam in magneto-optical memory using a magneto-optical recording medium. An optical system of a drive unit for the phase-change type optical information recording medium can be designed simpler than that for a magneto-optical recording medium, so that recently research and development of this kind of recording medium has been actively conducted.
As disclosed in U.S. Pat. No. 3,530,441, the so-called chalcogen-based alloys, such as Ge--Te, Ge--Te--Se, Ge--Te--S, Ge--Se--S, Ge--Se--Sb, Ge--As--Se, In--Te, Se--Te and Se--As, are conventionally used as recording materials for the phase-change type optical recording medium. In addition, it is proposed to add an element of Au to the above-mentioned Ge--Te based alloy to improve the stability and to increase the rate of crystallization of the recording material as disclosed in Japanese Laid-Open Patent Application 61-219692. Furthermore, the addition of Sn and Au to the Ge--Te based alloy, and the addition of Pd to the same are respectively proposed in Japanese Laid-Open Patent Applications 61-270190 and 62-19490 for the same purposes as mentioned above. Furthermore, recording materials comprising a mixture of Ge, Te, Se and Sb, and a mixture of Ge, Te and Sb are respectively disclosed in Japanese Laid-Open Patent Applications 62-73438 and 63-228433, each of which has a specific composition in terms of atomic percent of each constituent element of the recording material for the improvement of the recording and erasing repetition properties of the obtained recording medium.
However, none of the above-mentioned conventional phase-change type optical recording media satisfies all the requirements for the phase-change type rewritable optical recording medium. In particular, most important points to be improved in the conventional phase-change type optical recording media are to improve the recording sensitivity and erasing sensitivity, and to prevent the decrease of the erasability in the course of the overwriting operation, which is caused by the portions remaining unerased.
For the above-mentioned improvements, a mixed-phase type recording material comprising as constituent elements Ag, In, Sb and Te is proposed, as disclosed in Japanese Laid-Open Patent Applications 3-240590, 4-78031, 4-232779 and 5-345478. Such a mixed-phase type recording material can show the recording and erasing sensitivities to the light with a peak power of 12 mW or lees, and excellent erasabillty, so that it is effective when used in a recording layer for mark edge recording. However, conventional optical recording media employing the Ag--In--Sb--Te based recording materials are still insufficient with respect to the shelf stability and the repetition reliability.